Power circuit breakers conventionally utilize a trip actuator to provide a mechanical force which trips the circuit breaker in response to an electrical tripping pulse from over-current intelligence of the circuit breaker. A prior art trip actuator disclosed in U.S. Pat. Nos. 3,544,931 and 3,760,307 includes a permanent magnet disposed in a housing. A plunger is provided which has a first end disposed within the housing, and a second end outside the housing. The plunger is disposed for guided rectilinear movement. An actuating member is fixed to the external second end of the plunger, and the plunger includes a disc within the housing. The disc is acted upon by the magnet to provide a force which tends to move the plunger out of the housing, and the plunger is further acted upon by a spring which biases the plunger in a direction which tends to retract the plunger into the housing.
The prior art trip actuator further includes first and second levers, with the first lever having a first end disposed to be actuated by the plunger actuating member when the plunger retracts into the housing, and a second end which extends to a circuit breaker trip shaft. The breaker is tripped in response to actuation of the first lever. The second lever mechanically lifts the plunger, to reset the trip actuator, in response to movement of the movable contacts of the circuit breaker as they are propelled to the open position in response to tripping of the circuit breaker.
The magnet cannot pull and reset the disc against the force of the, spring acting on the plunger, but can overcome the spring force when the disc is in contact with the magnet pole piece which creates a low reluctance first magnetic circuit. An electrical tripping pulse from an over-current trip unit of the circuit breaker counter-acts the effect of the magnet, transferring the magnetic flux to a second magnetic circuit, which allows the spring to separate the disc from the magnet pole piece and move the plunger to actuate the trip shaft lever. The trip shaft lever then rotates the trip shaft and trips the breaker. As the breaker opens, a lever pin on the opening movable contact structure strikes a spring finger attached to the reset lever. This provides the assistance required to move the disc and close the air gap between the disc and the magnet, against the spring force. The device is reset when the disc contacts the magnet, re-establishing the first magnetic circuit, which has a lower reluctance than the second magnetic circuit.
While this prior art trip actuator performs well, it has a relatively high manufacturing cost, due in part to the precision required to provide a reliable rectilinear plunger movement which will operate correctly for the life of the breaker without skewing or binding. It would thus be desirable, and it is an object of the present invention, to provide a circuit breaker having a new and improved trip actuator which is less costly to manufacture than the hereinbefore mentioned prior art trip actuator, and which will operate as well as, or better, than the rectilinear type.